An electrophotographic apparatus uses a single-component or a two-component developer for electrophotography (hereinafter referred to as developer). The two-component developer may be composed of e.g., toner particles and carrier particles. An external additive is usually mixed to the developer. This external additive enables to impart to the developer a polarity opposite to or homologous to the polarity of the toner particles composing the toner. For instance, surface treated silica is adopted as external additive to give to the developer a negative electrostatic charge, i.e. a polarity opposite to the toner particles of a positive electrostatic charge. The external additive employed in order to give to the developer a positive electrostatic charge, i.e. the same polarity as the toner particles of the positive electrostatic charge, is for example aluminum oxide or the like.
Some external additives are constituted by an inorganic fine powder that, in addition to the above electrostatic characteristics, possesses conductive characteristics and prevents the toner from having an excessive electrostatic charge. Some other external additives serve the purpose of improving the flowability, controlling the electrostatic charge, or preventing the formation of a toner film on the surface of a photoreceptor. For example, the above silica is often composed of hydrophobic silica that is a surface treatment agent and improves the flowability of the developer. In addition, recent hydrophobic silica enables to stabilize the electrostatic characteristics of the developer through a special treatment, as disclosed in Japanese Publication for Unexamined Patent Application No. 45457/1983 (Tokukaisho No. 58-45457).
In addition to the above surface treatment agent made of hydrophobic silica, the external additive of some developers includes abrasive particles composed of cerium oxide, chromium oxide or the like, as for example disclosed in Japanese Publication for Unexamined Patent Application No. 81127/1978 (Tokukaisho No. 53-81127). Such developers enable to prevent the formation of a toner film on the surface of the photoreceptor, as the photoreceptor is polished by the external additive.
Furthermore, the flowability may be improved, the electrostatic charge controlled and the formation of a toner film may be prevented by mixing and adding various types of external additives to the developer, as disclosed in, e.g., Japanese Publication for Unexamined Patent Application No. 151952/1982, No. 40557/1983, No. 152257/1983, and 19667/1987 (Tokukaisho No. 57-151952, No. 58-40557, No. 58-152257, and No. 62-19667).
Namely, Japanese Publication for Unexamined Patent Application No. 151952/1982 (Tokukaisho No 57-151952) discloses a developer whose electrostatic charge is controlled by mixing 0.1 to 10 parts by weight of fine powder composed of a compound of SnO.sub.2 series as external additive, with 100 parts by weight of magnetic toner, to produce a single-component developer. The external additive mixed in the developer disclosed in Japanese Publication for Unexamined Patent Application No. 40557/1983 (Tokukaisho No. 58-40557) is a conductive fine powder composed of tin oxide, silver powder, nickel powder or the like, having a particle size of 0.01 to 0.5 .mu.m. This external additive permits to prevent the developer from causing an irregular development of solid areas and stains in the background.
The developer disclosed in Japanese Publication for Unexamined Patent Application No. 152257/1983 (Tokukaisho No. 58-152257) comprises an external additive produced by granulating with a binder at least one of various agents to produce particles of about 1 to 20 times the average particle size of the toner particles. The above various agents are, for example an electrostatic charge control agent composed of a quaternary ammonium salt, nigrosine dye or the like, an agent for improving the flowability and an abrasive. In the developer disclosed in Japanese Publication for Unexamined Patent Application No. 19667/1987 (Tokukaisho No. 62-19667), the electrostatic charge is controlled by adopting an external additive composed of a low molecular weight olefin polymer comprising an electrostatic charge control agent such as nigrosine dye or other agent.
As described above, in conventional developers, the flowability is improved, the electrostatic charge is controlled, and the formation of a toner film on the surface of the photoreceptor is prevented, by adding and mixing the above various external additives.
However, neither of the external additives adopted in conventional developers is capable of boosting the electrostatic charge of the toner, making it difficult to impart a sufficient electrostatic charge to the toner. As a result, the electrostatic charge of the toner does not rise in a satisfactory manner at the start of operations in the copying machine. This happens especially when the external additive is provided in small quantity.
Besides, with the developers disclosed in Japanese Publication for Unexamined Patent Application No. 152257/1983 (Tokukaisho No. 58-152257), and No. 19667/1987 (Tokukalsho No. 62-19667), the electrostatic charge control agent attached to the surface of the granulated external additive adheres to the surface of the carrier particles. The adhesion of the electrostatic charge control agent to the surface of the carrier causes the electrostatic charge of the toner to become insufficient when the carrier and the toner collide with each other, thereby shortening the life of the developer.
Accordingly, the addition and mixing of the above-mentioned external additives enable to improve the flowability of a conventional developer, but on the other hand cause the electrostatic charge or the rise of the electrostatic charge to be insufficient. Furthermore, when the external additive comprises an electrostatic charge control agent, the adhesion of the electrostatic charge control agent to the surface of the carrier shortens the life of the developer. Also, the use of a binder to disperse the electrostatic charge control agent causes the production cost to rise and the productivity to lower. Moreover, the above external additives are unable to prevent the toner from scattering whereby fog is liable to form in the background after a small number of copies are made, due to dust within the copying machine.